A light-emitting diode (LED) is a widely used semiconductor device whose main characteristic is that it will emit energy in the form of light, usually within the visible spectrum, when a current flows through the device. The basic mechanisms by which light-emitting diodes operate are well understood in this art and are set forth, for example, by Sze, Physics of Semiconductor Devices, 2d Edition (1981) at pages 681-703. Silicon carbide-based LEDs are described in U.S. Pat. Nos. 4,918,497 and 5,027,168, both of which are assigned to the assignee of the present invention and incorporated entirely herein by reference. Co-pending and commonly assigned application Ser. No. 08/081,668 filed Jun. 23, 1993 for "Blue Light-Emitting Diode with High External Quantum Efficiency" also sets forth exemplary information about such devices.
As is well known to those familiar with semiconductor devices, light-emitting diodes, and the interactions between light and matter, the wavelength of light (i.e., its color) that can be emitted by a given material from which a light-emitting diode is formed, is limited by the physical characteristics of that material, specifically its bandgap. The bandgap in a semiconductor material represents the amount of energy that separates a lower energy valence band and a higher energy conduction band in which carriers (electrons or holes) can reside in accordance with well-known principles of quantum mechanics. When electrons and holes travel across the bandgap and recombine, they will, under certain circumstances, emit energy in the form of light. Biasing a semiconductor p-n junction to produce a current flow is one way to obtain such recombinations and the visible light they emit. Because the wavelength of light is inversely proportional to its frequency, and its frequency is directly proportional to the corresponding energy transaction, certain wavelengths of light cannot be obtained in materials that have relatively narrow bandgaps. For example, blue light is generally considered to be that visible light which is emitted in the 400-500 nanometer (nm) portion of the visible spectrum. It will be understood that 400-500 nm is a somewhat arbitrary range, and that wavelengths close to 400 nm are also considered to be violet, and those close to 500 nm to be green. Such wavelengths require energy transactions of at least 2.6 electron volts (eV) which means that light-emitting diodes that will emit blue light must be formed of materials that have a bandgap of at least 2.6 eV. Such materials include, in certain circumstances, zinc selenide (ZeSe), gallium nitride (GAN), diamond, and silicon carbide (SiC).
The desirable theoretical characteristics of silicon carbide, and its potential as a source material for blue LEDs, have been well understood for a number of decades, dating back almost to the beginning of the semiconductor era. Nevertheless, the difficulties of working with silicon carbide have precluded most researchers from producing commercially successful devices from it. For example, the author of the article "Whatever Happened to Silicon Carbides?", IEEE Transactions on Industrial Electronics, Volume IE-29, No. 2, May, 1982, basically concluded that although silicon carbide was a theoretically interesting material, "I see no viable market for SiC semiconductor devices in the near future."
Silicon carbide has a number of attractive features from an electronic standpoint. It has a high saturated electron-drift velocity, a wide bandgap, a high thermal conductivity, a high breakdown electric field, and is "hard" to radiation. Silicon carbide presents difficulties, however, because it can crystalize in over 150 polytypes, many of which are separated by very small thermodynamic differences. As a result, and as well known to those familiar with crystal growth techniques of semiconductors and other materials, obtaining the necessary pure single crystals of silicon carbide, and the typical epitaxial or implanted layers that are generally desired or required in many semiconductor device structures, has long been a difficult task.
In recent years, however, the assignees of the present invention have made significant progress in taming the process difficulties presented by silicon carbide and in taking advantage of its desirable characteristics. These include success in the areas of sublimation growth of single crystals (e.g., U.S. Pat. No. 4,866,005); growth of epitaxial layers of silicon carbide on single crystals (U.S. Pat. Nos. 4,912,063 and 4,912,064); implantation and activation of dopants into silicon carbide (U.S. Pat. No. 5,087,576); and etching techniques for silicon carbide (U.S. Pat. Nos. 4,865,685 and 4,981,551).
Building upon these successes, the assignees of the present invention have produced the first commercially viable blue light-emitting diodes in significant commercial quantities at reasonable prices. Such LEDs are thoroughly described in U.S. Pat. Nos. 4,918,497 and 5,027,168 which, as noted above, are incorporated entirely herein by reference.
Because an LED is a diode structure (i.e., a p-n junction), commercial LEDs formed from silicon carbide generally include an n-type substrate and terminate in a p-type epitaxial layer, or alternatively, incorporate a p-type substrate and terminate in an n-type layer. The characteristics of silicon carbide, however, are such that the n-type of silicon carbide is somewhat easier to dope, is more transparent when doped, and, as is usually the case with n-type semiconductors, has a greater conductivity than p-type silicon carbide. Accordingly, the use of n-type silicon carbide layers wherever possible affords greater electrical conductivity (lower resistance) and optical transparency with resulting increases in light emission, efficiency, and current spreading for LED structures made therefrom.
To date, however, the art has lacked a technique for maximizing the use of n-type silicon carbide in p-n junction LEDs. For example, although the nature of n-type silicon carbide is such that it would be advantageous to use it for both a substrate and a top layer of an LED, the presence of a p-n junction between two n-type layers would essentially result in an n-p-n structure; i.e., a bipolar junction transistor. As is known to those of ordinary skill in this art, a transistor functions quite differently from an LED and thus such a structure has to date remained impractical and undesirable.